Many methods for fractionating components of a feedstock fat are known, including methods of dry fractionation and methods of solvent fractionation. The steps of the methods of dry fractionation, which do not use any solvent, are relatively simple. However, the methods have drawbacks concerning the low purity of the fat component of interest or the difficulty in operating the fractionation steps. The methods of solvent fractionation advantageously overcome the above-mentioned drawbacks of the methods of dry fractionation, but have other drawbacks such as a long operation time due to an increased cooling load resulting from a high concentration of a fat dissolved in a solvent.
JP-B 38-917, for example, describes a method of solvent fractionation-crystallization of a fat comprising treating a fat in a rapid heat exchanger. This method is different from the method of the present invention in that the pre-cooling is performed below the crystallization temperature. Although the method of the publication is not defined by the yield of crystals based on the total amount of the fat at the crystallization temperature of the feedstock fat, the yield of around 8% described in Examples of the publication is quite different from that of 20% or higher obtained by the method of the present invention. The ratio of the total cooling load to the load for removing heat of crystallization in a crystallizer obtained by the method of the publication is 1, which is different from that obtained by the method of the present invention. The method of the publication is also different from the method of the present invention in that the method is not defined by the temperature of a refrigerant used in the crystallization.
When a feedstock fat which yields 20% or higher of crystals based on the total amount of the fat at the crystallization temperature is crystallized according to the method described in JP-B 38-917, deposition of crystals occurred before the temperature reached the crystallization temperature even if a rapid cooling was applied. Introduction of the feedstock fat in such a state into a crystallizer resulted in deposition of unnecessary components as constituents of crystals. Accordingly, a product with the objective quality was not obtained. These results are construed as follows. If a feedstock fat which yields 20% or higher of crystals is crystallized, a step of crystal growth should be usually performed slowly in order to avoid the formation of crystals before reaching the crystallization temperature. However, the step was performed rapidly. As a result, the incorporation of unnecessary components etc. occurred, which resulted in a failure in obtaining a product with the objective quality.